Microsoft 365 Error Codes

Fix Microsoft 365 Email Delay SMTP Error 451 4.7.500

Maira Mehtab Sep 28, 2026 869 views

Question details

Users experience delayed emails to Microsoft 365 recipients, accompanied by the SMTP error code 451 4.7.500 Server busy.

Product: Microsoft 365
Device & OS: not provided
Scenario: Sending emails to Microsoft 365 recipients after a recent change in the sender's outgoing IP address or sending patterns.
Observed behavior: Outgoing emails are delayed for several hours, and the sender receives a non-delivery report (NDR) with the error '451 4.7.500 Server busy' due to graylisting and IP-reputation filtering.

Before you start

Ensure you have access to your email provider's outgoing configuration details and know the IP addresses recently used to send emails.

Solution 1Recommended

Verify IP Reputation and Email Authentication

Work with your email provider to establish a clean sending history and verify your IP reputation.

This error usually occurs due to graylisting when Microsoft detects a significant change in your source IP address. New IP addresses are treated with suspicion until they build an established sending pattern.

Check IP reputation

Contact your third-party email provider or network administrator to verify that your newly assigned outgoing IP address is not on any spam blocklists.

Verify email authentication

Ensure that your SPF, DKIM, and DMARC records are correctly configured and updated to include the new outgoing IP address.

Wait for automatic recovery

Continue normal, legitimate mailing activities. Microsoft 365 often resolves this graylisting issue automatically after a few days as your new IP address establishes a consistent sending history.

Automatic Recovery Timeframe: There is no guaranteed exact timeframe for automatic recovery. Building a positive IP reputation takes time depending on your email volume.

Solution 2

Configure an Exchange Online Connector

A workaround for Microsoft 365 recipients to allow emails from the affected sending server.

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Frequently Asked Questions

What does SMTP error 451 4.7.500 mean?

The '451 4.7.500 Server busy' error indicates that Microsoft 365 has temporarily delayed emails from your server. This usually happens as part of graylisting or IP-reputation filtering when a significant change in your sending IP address or pattern is detected.

How long does it take for Microsoft 365 to fix the IP reputation issue?

There is no guaranteed exact timeframe. Typically, if you are using a legitimate third-party service or on-premises Exchange, the graylisting issue often resolves automatically after a few days once an established, safe sending pattern is built.

Can I bypass the 451 4.7.500 error immediately as a sender?

As a sender, you cannot bypass it instantly because Microsoft controls the receiving server's reputation checks. However, the Microsoft 365 recipient can create an Exchange Online inbound connector as a workaround to allow traffic from your specific IP address.

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